The invention relates to a search radar apparatus containing an MTI video processing unit provided with a canceller for generating video signals of moving targets, a zero-velocity filter for generating clutter video signals, and clutter level indication means for determining a standard clutter level per range-azimuth clutter cell of the radar range, using clutter video signals.
Such a search radar apparatus is known from IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-15, No. 4, July 1979, pp. 508-516, where the video signals passed through the zero-velocity filter and exceeding a certain threshold are selected for the processing of these signals by the clutter level indication means, containing a recursive filter and a clutter memory connected thereto. The utilization of a zero-velocity filter with a substantially rectangular filter characteristic has the advantage that the video signal selection occurs purely on a frequency basis. It is however a costly matter to manufacture such a filter; moreover, it is difficult to realize. The use of a more economical zero-velocity filter, whose filter characteristic deviates considerably from the rectangular shape, requires a clutter signal selection not on frequency exclusively, but on frequency and amplitude. In this case, it is possible that a video signal of a certain amplitude and at a certain doppler frequency situated in the first stop band of the filter characteristic, has a filter response below the threshold value, whereas a video signal of a much greater amplitude and a slightly higher frequency in the first stop band has a filter response exceeding this threshold value. A video signal selection made in this way will result in an incorrect indication of the ground clutter and quasistationary targets present; this must be considered as a disadvantage of the search radar apparatus containing such an MTI video processing unit.